1. Field of the Invention
This invention relates to an amplification arrangement for processing a plurality of individually generated desired signals.
2. Description of the Prior Art
The problem which is frequently encountered in amplification technology, namely that of amplifying and further processing a low level desired signal which is superimposed by a substantially stronger spurious signal--particularly frequently a supply line disturbance--is conventionally solved by the use of a differential amplifier with common mode rejection. This solution can relatively easily be implemented without running into problems when two desired signals having superimposed thereon spurious signals of the same magnitude are applied to the differential amplifier without distortion, which means when a spurious signal of the same magnitude is applied to the input of the differential amplifier.
However, serious problems arise when the individual signals must be amplified prior to the difference check (difference formation) and when the output impedance should be lowered. Such problem exists, for example in connection with physiological measurements, such as in connection with EEG measurements, in connection with which the very weak desired signal can be superimposed by a spurious signal which is stronger by several orders of magnitude. Then, extremely high requirements prevail as far as the preamplifier for each input signal is concerned. A very small difference between amplification factors of the input amplifiers has the consequence that spurious signals of different strengths appear at the output terminals of these amplifier stages. The differential spurious signal resulting from these differences is then no longer suppressed by the common mode rejection of the succeeding differential amplifier, but rather, it is amplified as if it were a desired signal.
In order to solve this problem, it is known to use as a reference potential for the amplifier arrangement not the zero potential of the circuit (for example mass) but the level of the noise signal. As a result of this principle, the spurious voltage which is applied to the inputs as a push-push voltage is not amplified, but rather appears at the output with identical magnitude. The non-amplified spurious voltage appearing at the output terminals must then be suppressed by a succeeding differential stage. An example of such circuitry involving several single amplifiers is described in DE-OS No. 24 29 953 (published German patent specification).
The common mode rejection in the differential amplifier--or differential amplifier stages--has the disadvantage that at least one additional difference-forming stage is needed in addition to the preamplifier or preamplifiers; the desired signal, without the spurious signal, is obtained only then at the output of such stage. For many measuring applications, particularly in physiological measuring technology such as for EEG and EKG measurements, it is, however, desirable to have the individual useful signals available directly at the output of the preamplifier, in order to make use of various signal processing possibilities without having to proceed via a difference-forming stage or via a differential amplifier stage.